Recently, an UPS of three-phase four-wire high-frequency link has been proposed in some controlling strategies, which include peak value current control and mean value current control, etc. With such improved controlling strategies, the content of the quintic or septic harmonic waves and the like can be significantly reduced. Although these controlling strategies may effectively improve the input power factor, they have their respective drawbacks. In case of the peak value current controlling strategy, the wave shapes of the three-phase input currents are different from each other, and the overall current of the harmonic waves is large. The mean value controlling strategy is widely used and has a smaller overall current of harmonic waves in comparison with the peak value controlling strategy. However, a drawback of this strategy is that the input voltage is restricted and the switching frequency cannot exceed 20 KHz in high power applications.
FIG. 1 shows an uninterrupted power supply system using a mean value injection control, which comprises AC input ends U, V and W, a rectifying circuit, a battery branch, a power factor correction branch and an inverter branch. The input end of the rectifying circuit is connected with the AC input ends U, V and W, while the input end of the inverter branch is connected in parallel with the positive and negative ends on the DC side of the power factor correction branch. The power factor correction branch is placed between the battery branch and the inverter branch, and it includes a positive and negative boost converter circuit, a generating circuit of driving signals for power factor correction and positive and negative DC bus capacitors C1 and C2. The positive and negative boost converters include inductors L1, L2, L4 and L5, forward diodes D1 and D4, and positive and negative boost converter switch tubes Q9–Q12 respectively. The inductors L1, L2, L4 and L5 of the positive and negative boost converters are connected in series with the diodes D1–D4 respectively on the positive and negative DC bus. The positive DC bus capacitor C1 and the positive boost converter switch tubes Q9 and Q10 are cross-connected on the positive DC bus and the neutral line and placed before and after the diode D1 of the positive boost converter respectively. The negative DC bus capacitor C2 and the negative boost converter switch tubes Q11 and Q12 are cross-connected on the negative DC bus and the neutral line, and placed before and after the diode D4 of the negative boost converter. The input ends of the generating circuit of driving signals for power factor correction are connected with the positive and negative output voltage sampling signals REC1 and REC2 after the input voltage is rectified, the feedback signals V1 and V2 of the positive and negative DC bus capacitor voltage, and the sampling signals I1 and I2 of the induced current of the positive and negative boost converter respectively, while its output ends are connected with the control ends of the switch tubes Q9 and Q11 of the positive and negative boost converter. The positive and negative boost converter is formed with two symmetrical boost converter circuits in parallel, each of which includes an inductor (L1 or L2 or L4 or L5), a forward diode (D1 or D2 or D3 or D4) and a switch tube (Q9 or Q10 or Q11 or Q12). A driving signal shifting circuit is provided after the generating circuit of driving signal for power factor correction for regulating the phase of the driving signal. The signals from its input end and output end are sent to the controlling ends of a first positive boost converter switch tube (Q9), a first negative boost converter switch tube (Q11), and a second positive boost converter switch tube (Q10), a second negative boost converter switch tube (Q11), respectively.
However, there is only replacement without holding the width of the Pulse it may result in uneven current of those two voltage increase converters, and thus the system reliability is low.